Alzheimer's disease (AD), a progressive neurodegenerative disorder of the elderly, is characterized by the deposition of 13-amyloid (AI_) and neurofibriUary tangles in the hippocampus and cerebral cortex. Endoproteolytic cleavages of amyloid precursor protein (APP) by _-and ?- secretases result in the generation of AI3 peptides that are believed to be neurotoxic. The presenilins (PS1 and PS2), which when mutated cause familial AD, are important for the intramembraneous proteolysis of several proteins, including APP and Notch1. The presenilins is part of a high molecular weight complex that contains the ?-secretase. Critical to this 7-secretase complex is another type I transmembrane glycoprotein, termed nicastrin that binds to both APP and Notch1 and is required for glp-l/notch signaling. Recent studies in nicastrin-deficient Drosophila demonstrate that the loss of nicastrin abolishes the presenilin-dependent intramembraneous proteolysis of Notch. Thus, we hypothesize that nicastrin is required for proteolytic processing and signaling of Notch1 in mammals. We plan first to determine whether reduced levels of nicastrin impact on presenilin-mediated Notch signaling in mammals by generating nicastrin.deficient mice and characterize the phenotypic consequences of reduction in nicastrin. Secondly, because studies indicated that nicastdn plays an important role in APP processing, we will test the role of nicastrin in presenilin-mediated 7-secretase cleavage of APP using nicastrin knockout cells. In addition, we will determine the mechanism whereby nicastrin influences presenilins to facilitate transmembrane cleavage of Notch and APP. Finally, to examine the in vivo role of nicastrin in adult brain, we will generate and characterize nicastrin conditional knockout mice. Taken together, outcomes from these efforts will have important implications toward our understanding of the mechanism whereby nicastrin influences presenilins in facilitating the presenilin-dependent 7-secretase activities on several critical pathways, including Notch and APP, as well as for nicastrin as a potential therapeutic target in AD.